Preheat oven to 350 degrees F, with racks in upper and lower thirds. Line two 24-cup mini muffin pans with paper liners. In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, using an electric mixer, beat together peanut butter, butter, and brown sugar on high until pale and fluffy, about 3 minutes. Beat in egg and egg yolk, scraping down bowl as needed. With mixer on low, beat in flour mixture, buttermilk, and vanilla until combined.

With the large end of a melon baller or a spoon, place 2 teaspoons batter into each muffin cup, then press a peanut-butter candy into each center until batter aligns with top edge of candy. Bake until puffed and set, about 10 minutes, rotating pans halfway through. Immediately place a piece of candy corn on top of each cupcake; let cool completely in pans on wire racks.